The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS system and evolved UTRAN (e-UTRAN) is the radio access network of an LTE system. As illustrated in FIG. 1, an e-UTRAN typically comprises user equipments (UE) 150 wirelessly connected to radio base stations (RBS) 110a-c, commonly referred to as eNodeB. The eNodeB serves one or more areas referred to as cells 120a-c. In FIG. 1 the UE 150 is served by the serving cell 120a. Cells 120b and 120c are neighboring cells. In e-UTRAN the eNBs 110a-c are directly connected to the core network and may communicate with each other over the X2 interface. In a UTRAN however, the RBSs or NodeBs (NB) are connected to the core network via a Radio Network Controller which controls the NBs connected to it.
There are several reasons for why synchronized RBSs are used. In Time-Division Duplex (TDD) systems such as TDD-LTE, TDD-UMTS and Time Division Synchronous Code Division Multiple Access (TD-SCDMA) where time-division multiplexing is used to separate uplink and downlink signals, the synchronization is mandatory. The NBs need to be synchronized with a certain predefined frequency and phase accuracy. The requirement for phase alignment is less than 3 μs and the frequency accuracy should be within 50 ppb. In Frequency-Division Duplex (FDD) systems such as FDD-LTE and FDD-UMTS, where different frequencies are used for uplink and downlink signals, time synchronization is optional. However, the synchronization is necessary for the performance of certain services and algorithms, such as Multicast Broadcast Single Frequency Networks (MBSFN) and inter-cell interference coordination. In general, synchronized radio networks allow for a higher capacity in the network.
According to the 3GPP standard, the inter-RBS air time synchronization is achieved by a Global Positioning System (GPS) satellite. A signal from the satellite indicates the GPS time which provides an absolute timing reference. The GPS time may e.g. be used to synchronize the frame time over the air and thereby an inter-RBS air time synchronization may be achieved.
A problem with using GPS for the synchronization is that the GPS signal may be unavailable due to e.g. bad weather or due to a faulty GPS receiver. However, the RBS will still need to provide service to UEs, preferably until the GPS signal is recovered. Typically, the requirement on the operator of the network is to be able to provide the service to the users at least during 24 hours of GPS unavailability. This is approximately the time needed to receive a notification of the faulty GPS receiver and to send someone to replace it. The sustainability of the RBS service during the absence of the GPS signal is a very important characteristic of the system, especially for TDD systems where the synchronization is mandatory for a functioning network. For FDD systems, it is important for the maintenance of a high system capacity and for certain services.
One solution to keep phase stability when no GPS signal can be received is to use an Oven mounted Voltage Controlled Crystal Oscillator (OVCXO) which can maintain very high frequency stability at the absence of a time reference signal during a longer time than an ordinary oscillator. An OVCXO would enable maintenance of a stable phase (+/−1.5 μs) without the GPS signal during 24 hours, while an ordinary oscillator that is commonly used in the RBS maintains phase stability during around 30 minutes. The drawback of the OVCXO is that it is very expensive.
Another possible solution to improve phase stability when no GPS signal can be received is to use a grand master clock and distributed slave clock concept, e.g. according to IEEE 1588 specifications. However, this solution is based on a special packet exchange procedure to compensate the time the packet takes to traverse the network, and as the packet latency will depend on the network load, the timing accuracy of such a solution cannot be assured. Another drawback is that network elements that do not support the solution will degrade the timing accuracy achieved in the slave clock.